What is your budget for a budget bike computer?
The biggest decision I'd think is whether you want to only see your data, speed, distance, time, etc., while you are on the bike only or do you want to be able to review your ride in more detail on your computer or phone after the ride?
Some devices are very proprietary and only work with their software. Others more popular brands will work with many different websites like RideWithGPS, STRAVA and many others or with software on your machine like Golden Cheetah. (or is it cloud based now?)